In recent years, a technique of making thin film transistors (TFTs) using a semiconductor thin film (on the order of several to several hundreds nanometers in thickness) formed over a substrate having an insulating surface has been attracting attention. The thin film transistors are widely applied to electronic devices such as ICs and electro-optical devices, and have been quickly developed in particular as switching elements for image display devices.
In particular, active matrix display devices (such as liquid crystal display devices or light-emitting display devices) in which a switching element of a TFT is provided on each display pixel arranged in matrix, have been actively developed.
In addition, in order to reduce manufacturing costs, developments for forming a driver circuit area and a pixel area on the same substrate have also been advanced. Above all, a TFT using a polysilicon film has higher field-effect mobility than a transistor using an amorphous silicon film, and thus can operate at a higher speed.
In a module that is mounted on the display device, a pixel area that displays images for each function block and a driver circuit for controlling the pixel area, such as a shift register circuit, a level shifter circuit, a buffer circuit, and a sampling circuit, which are CMOS circuits in many cases, are formed over one substrate.
In particular, for a light-emitting display device that has organic light-emitting elements arranged in a matrix, a plurality of transistors having different roles is required for one pixel. In addition, also for a liquid crystal display device, an attempt to form switching transistors and a memory element such as an SRAM in one pixel has been made.
In Reference 1 (Japanese Patent application Laid-Open No. 2001-013893), a transistor that has a multi-gate structure is used in an EL display device.
Generally, current flowing between a source region and a drain region (in a channel region) is controlled by voltage applied to a gate electrode. When the channel length is large enough, in the case where the voltage applied to the gate electrode has a certain value (threshold value) or less, current hardly flows in the channel region. Subsequently, when voltage applied to the gate electrode exceeds the threshold value, current flowing in the channel region increases almost linearly.
As described above, when the channel length is large enough, the threshold is almost constant; however, when the channel length is small, current flows even when voltage of the threshold or less is applied to the gate electrode. This results from reduction of voltage barrier at a boundary between the source region and the channel region due to drain voltage, which is caused as the channel length is shortened. This phenomenon means that as the channel length is shortened, the threshold voltage is reduced, and is known as a typical example of a short channel effect.
When the short channel effect occurs in any channel region of a transistor having a multigate structure, characteristics of the transistor are impaired. In particular, when the short channel effect occurs in a source region adjacent to a source electrode, characteristics of the transistor are impaired.